Method for estimating concentration of generating hotochemical ozone by the use of pump-probe method, and apparatus for estimating concentration of generating photochemical ozone utilizing the method

ABSTRACT

A method of estimating strength of forming photochemical ozone utilizing a pump and probe technique is disclosed along with an apparatus for estimating strength of forming photochemical ozone utilizing a pump and probe means whereby strength of forming photochemical ozone can be estimated without measuring a concentration of each type of various reactive hydrocarbons in order to predict strength of forming photochemical ozone from rates of their reaction with OH radicals. Atmosphere ( 11 ) is irradiated with a pumping laser light ( 2   c ) to photolyze ozone therein, thereby forming excited oxygen atoms O ( 1 D) which are then caused to react with water vapor to form OH radicals. The OH radicals are irradiated with a probing laser light ( 3   e ) to excite electrons therein. A fluorescent light emitted when the excited electrons are relaxed to transition is measured in a time sequence. The intensity of fluorescence being proportional to the concentration of the OH radicals, the strength of forming photochemical ozone can be estimated from the change of the concentration of the OH radicals with time.

TECHNICAL FIELD

[0001] The present invention relates to a method of and apparatus forestimating a concentration or strength of forming photochemical ozoneutilizing a pump and probe technique.

BACKGROUND ART

[0002] In urban areas where human activities are active, there arevolatile organic compounds (VOCs) emitted notably in addition to theemissions of NO_(x), SO₂ and carbon monoxide. These VOCs are emittedmuch more in urban areas than in the other areas. As for the VOCs,reactive hydrocarbons which react with OH radical are determinative ofthe strength of forming photochemical ozone in an urban area. While thereactive hydrocarbons are divergent in types, including methane,non-methane hydrocarbons (NMHCs), alcohols, and carbonyl compounds(ketones, aldehydes), the NMHCs which are highly reactive are especiallyimportant as being precursors of ozone.

[0003] The increase in the amount of emission of NMHCs as precursors ofozone in urban areas in recent years is not only recognized toaccelerate the problem of air pollution in those areas but also is nowpressingly to raise the important issue of adversely affecting theglobal atmospheric environment.

[0004] The strength or concentration of air pollutants has so far beenmeasured using sensors which measure the individual concentrations ofthese NOx, SO₂, CO and VOCs respectively.

[0005] As for NMHCs, however, it is considered that there exist at least200 types which are counted from an olefin to aromatic hydrocarbons, andmeasuring the individual concentrations of all these reactivehydrocarbons to estimate their rates of reaction with OH radicals and toestimate from their each rates the concentration or strength of ozone tobe generated photo-chemically would give rise quite possibly to a largeerror despite the large efforts which this estimation should entail.

DISCLOSURE OF THE INVENTION

[0006] With the aforementioned prior-art problem taken into account, thepresent invention has for its objects to provide a method of and anapparatus for estimating strength of forming photochemical ozone or aconcentration of ozone to be formed photo-chemically in an atmosphere,by a pump and probe technique whereby the strength of formingphotochemical ozone can be estimated without the need to measure all ofa concentration of each type of various reactive hydrocarbons in orderto estimate from their rates the concentration of ozone to be generated.

[0007] In order to achieve the object mentioned above, there is providedin accordance with the present invention a method of estimating strengthof forming photochemical ozone utilizing a pump and probe technique,characterized in that it comprises the steps of: irradiating atmospherecontaining air pollutants with a pumping laser light to form OH radicalstherefrom; irradiating such formed OH radicals with a probing laserlight to cause a fluorescent light to be emitted therefrom; measuring achange of intensity of the fluorescent light with time; and determininga concentration of the air pollutants from the measured change ofintensity of the fluorescent light with time.

[0008] In the method mentioned above, preferably the said pumping laserlight is a pulsed laser light having its wavelength adapted to photolyzeozone in the atmosphere, thereby forming excited oxygen atoms O (¹D),and the said OH radicals are formed by the reaction of the excitedoxygen atoms O (¹D) with water vapor. Also, the said probing laser lightis preferably a pulsed laser light having its wavelength adapted toexcite electrons in the OH radicals.

[0009] In the method mentioned above, the said change of intensity ofthe fluorescent light with time may be measured by irradiating the saidpumping laser light and then irradiating repetitively the said probinglaser light each with a given time interval, and measuring the intensityof the fluorescent light after each time the said probing laser light isirradiated.

[0010] According to the forming photochemical ozone strength estimatingmethod utilizing the pump and probe technique as mentioned above, thepumping laser light is used to photolyze ozone in the atmosphere andthereby to form excited oxygen atoms O (¹D) which are then permitted toreact with water vapor to form OH radicals. Then, irradiating suchformed OH radicals with the probing laser light excites electrons in theOH radicals, the excited electrons being relaxed to emit a fluorescentlight whose intensity is proportional to the concentration of the OHradicals. The OH radicals are iteratively irradiated with such a probinglaser light with a given time interval, and the intensity of such afluorescent light is measured at each time after when OH radicals areirradiated with a probing laser light, whereby a change of theconcentration of the OH radicals with time are found.

[0011] Since the reactive hydrocarbons in the atmosphere consume OHradicals by reacting with them, the change of the concentration of OHradicals with time increases in proportion with the concentration of thereactive hydrocarbons. It is thus possible to estimate the strength offorming photochemical ozone in the atmosphere from the measured changeof the concentration of OH radicals with time, without the need toseparately measure a concentration of each type of the various reactivehydrocarbons and then to predict from their concentrations the rates oftheir individual reactions with OH radicals in order to predict astrength of forming photochemical ozone.

[0012] The present invention also provides an apparatus for estimatingstrength of forming photochemical ozone utilizing a pumping and aprobing means, characterized in that it comprises a pumping laser lightoscillator means; a probing laser light oscillator means; a OH radicalforming means; a probing laser light lead-in means; a fluorescencedetection means for measuring an intensity of fluorescence; and acontrol means for providing the pumping laser light oscillator means,the probing laser light oscillator means and the fluorescence detectionmeans with timing control signals whereby they are co-operated.

[0013] According to this apparatus makeup with these means operated bysuch timing control signals, atmosphere flowing within the radicalforming means for forming OH radicals can be irradiated with a pulsedlaser light to form OH radicals in the atmosphere, the OH radical in theatmosphere can be irradiated with a pulsed probing laser lightintroduced through the laser light lead-in means to emit a fluorescentlight in the fluorescence detection means, and the intensity of thefluorescent light can be collected by a condensing mirror and condensinglenses and can then be measured by a photo detector in the fluorescencedetection means.

[0014] In this apparatus, the said radical forming means preferablyincludes: a first and a second straight tube each connected to a sidewall of said fluorescence detection means extending coaxially, and thesaid first straight tube having an outer end sealed hermetically with atransparent window and having an air inlet port in a region of the outerend of the said first straight tube, the said second straight tubehaving an outer end sealed hermetically with a transparent window andhaving an air out let port in a region of the outer end of the saidsecond straight tube, whereby the atmospheric air can be led into theradical forming means, and the pumping laser light, e. g., pulsed, canbe introduced into the radical forming means, passing, in the first andsecond straight tubes axially thereof wherein OH radicals are allowed toform. Further, the said radical forming means is preferably provided atthe said air outlet port with a flow control means and a vacuum pumpwhereby the atmospheric air introduced into the radical forming means isallowed to flow in the first and second straight tubes at a controlledrate of flow.

[0015] According to this specific apparatus makeup, the flow controlmeans in the air outlet port allows the atmospheric air to flow in alaminar flow optimum for the measurement and hence makes the measurementaccurate. Further, the straight tubes allow the atmosphere to be sealedhermetically therein with the transparent windows provided at theiropposed ends and also allow a pumping laser light to be introducedaxially of the tubes so as to form OH radical reproducibly and to passtherethrough so as not to produce a stray light in the apparatus.

[0016] The said probing laser lead-in means may include: a first and asecond straight tube each connected to a side wall of said fluorescencedetection means extending coaxially, and each of the said first andsecond straight tubes having an outer end sealed hermetically with atransparent window and having a plurality of baffle plates, whereby theprobing laser light, e. g., pulsed, when introduced into the first andsecond straight tubes may cause the OH radicals to be excited. Accordingto this specific apparatus makeup, a pulsed probing laser light can beintroduced axially of the tubes so as to excite electrons in the OHradicals reproducibly and can be passed there-through so as not toproduce stray light in the apparatus. Further, the straight tubesprovided with the baffle plates prevent the propagation of even a smallstray light and hence make it possible to effect the measurement with anincreased sensitivity.

[0017] The said fluorescence detection means may include: a condensingmirror and convex lenses means for collecting a fluorescent lightgenerated in an area in which the first and second straight tubes of thesaid radical forming means and the first and second straight tubes ofthe said probing laser light lead-in means intersect axially, a photodetector for measuring an intensity of the collected fluorescent light,and a receptacle composed of a nontransparent material for retaining thesaid condensing mirror and convex lens means and the said photo detectorin a sealed state. According to this specific apparatus makeup in whichthe condensing mirror and convex lenses means is used to collect thefluorescence emitted from a narrow area, it is possible to detect thefluorescence substantially entirely and hence to effect the measurementwith an increased sensitivity and with due precision.

[0018] The said control means preferably includes a system clock meansfor providing a first timing signal whereby the said pumping laser lightoscillator means is operated to issue a pulsed laser pumping light, asecond timing signal timed on the basis of the first timing signalwhereby the said probing laser light oscillator means is operated toissue a pulsed laser probing light repetitively with a given timeinterval, and a third timing signals whereby measurement by the saidphoto detector is synchronized with each pulsed laser probing lightissued. According to this specific apparatus makeup, the timing can beaccurately taken each of the OH radical formation, the electronsexcitation and the fluorescence detection, which assures a reproduciblemeasurement with due sensitivity and accuracy.

[0019] The said pumping laser light oscillator means is preferablyadapted to issue a pulsed laser light having a wavelength of 266 nmrepetitively at a repetition rate of 1 Hz. According to this specificapparatus makeup, a pulsed pumping laser light having a wavelength of266 nm is irradiated to atmosphere to photolyze ozone therein to formexcited oxygen atoms O (¹D) which are then allowed to react with watervapor to form OH radicals. The repetition cycle in which such a pulsedpumping laser light is iteratively applied to the atmosphere is as shortas 1 Hz, thus reducing the measurement time period.

[0020] Also, the said probing laser light oscillator means is preferablyadapted to issue a pulsed laser light having a wavelength of 281.9 nmrepetitively at a repetition rate of 1 kHz. According to this specificapparatus makeup, the electron excitation of OH radicals can be effectedefficiently. The repetition cycle in which such a pulsed probing laserlight is iteratively applied to the atmosphere is as short as 1 kHz,permitting the electron excitation to be effected a number of times in atime interval between successive irradiations of a pulsed pumping laserlight and hence the change of concentration of OH radicals with time tobe measured highly accurately.

[0021] The said photo detector preferably comprises a PMT(photomultiplier tube). According to this specific apparatus makeup,high amplification factor of the PMT assures a high sensitivity of themeasurement.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention will better be understood from thefollowing detailed description and the drawings attached hereto showingcertain illustrative forms of embodiment of the present invention. Inthis connection, it should be noted that such forms of embodimentillustrated in the accompanying drawings hereof are intended in no wayto limit the present invention but to facilitate an explanation andunderstanding thereof. In the drawings:

[0023]FIG. 1 shows a diagram illustrating the makeup of an apparatus formeasuring the concentration of air pollutants utilizing a pump and probetechnique in accordance with the present invention; and

[0024]FIG. 2 represents a diagram illustrating a control system for theapparatus for measuring the concentration of air pollutants utilizingthe pump and probe technique in accordance with the present invention

BEST MODES FOR CARRYING OUT THE INVENTION

[0025] Hereinafter, the present invention will be described in detailwith reference to several forms of implementation thereof illustrated inthe drawing figures.

[0026] At the outset, mention is made of a mechanism in which ozoneforms or is generated photo-chemically. Generating ozone (the principalcomponent of photochemical oxidant) in the troposphere requires thepresence of nitrogen oxide (NO_(x)) and carbon monoxide or a hydrocarbon(methane or a non-methane hydrocarbon). First, ozone existing in theatmosphere absorbs the energy of a solar UV (with a wavelength shorterthan 300 nm) and is photo-decomposed to form an oxygen molecule (O₂) andan oxygen atom in its excited state O (¹D) in accordance with theequation:

O₃+UV→O₂+O(¹D)  (1)

[0027] By the oxygen atom in the excited state O (¹D) is meant an oxygenatom with an electron therein in the excited state.

[0028] Such excited oxygen atoms thus formed are highly reactive andreact with water vapor to form OH radicals in accordance with theequation:

O(¹D)+H₂O→2OH  (2)

[0029] The OH radicals are highly reactive and react with variouschemical species. Since the chemical species that can react with the OHradicals which exist in a relatively clear atmosphere are limited tocarbon monoxide or methane, carbon monoxide and methane here react withOH radicals in a relatively clean atmosphere (a suburban or remotearea). The OH radicals react with carbon monoxide as follows:

OH+CO→CO₂+H  (3)

[0030] to form hydrogen atom (H) radicals. The hydrogen atom radicalsreact with oxygen molecules abundantly existing in the atmosphere asfollows:

H+O₂→HO₂  (4)

[0031] to form HO₂ radicals.

[0032] If OH radicals react with methane, the reactions:

OH+CH₄→H₂O+CH₃  (5)

CH₃+O₂→CH₃O₂  (6)

[0033] follow to form per-oxy radicals (methyl per-oxy radical: CH₃O₂).These radicals may react with NO (the main component of nitrogen oxides:NO_(x)) if it exists in the atmosphere as follows:

CH₃O₂+NO→NO₂+CH₃O  (7)

[0034] and through the reactions:

CH₃O+O₂→CH₃OO₂  (8)

[0035] there are formed CH₃OO₂ radicals, which bring about monomoleculardecomposition reaction:

CH₃OO₂→H₂CO+HO₂  (9)

[0036] to form formaldehyde H₂CO and HO₂ radicals.

[0037] In this way, ozone in the atmosphere by absorbing the ultravioletlight is decomposed to form excited oxygen atoms O (¹D), which reactwith water vapor to form OH radicals. The OH radicals thus formed areconsumed by reacting with air pollutants, e. g., carbon monoxide and/ormethanol.

[0038] Next, mention is made of the operations of HO₂ radicals formed asa result of the reaction of OH radicals with carbon monoxide ormethanol.

[0039] HO₂ radicals finally formed by the reaction of OH radicals withcarbon monoxide or methane react with NO as follows:

HO₂+NO→NO₂+OH  (10)

[0040] whereby they themselves are returned to OH radicals while NO isconverted into NO₂, which reacts with the solar light as follows:

NO₂+UV→NO+O(³P)  (11)

[0041] to form NO and oxygen atoms in the ground state O (³P), thelatter forming ozone as follows:

O(³P)+O₂→O₃  (12)

[0042] Thus, as long as the solar light exists, OH is converted to HO₂and then returns to OH while NO is converted to NO₂ and then returns toNO, and the reactions expressed by equations (3) to (11) are cyclicallyrepeated a number of times. Ozone is generated as a by-product and isaccumulated. This is the atmospheric photochemical theory (theorem).

[0043] As for the urban atmosphere in which there exists an abundance ofnon-methane hydrocarbons (NMHCs) emitted by human activities, not onlyare there reactions of OH radicals with carbon monoxide and methane, butalso there are their reactions with NMHCs. If the NMHC is a saturatedhydrocarbon, then the reactions:

NMHC+OH→H₂O+R  (13)

R+O₂→RO₂  (14)

[0044] take place, forming alkyl peroxy radicals RO₂ as in the equation(8). If the NMHC is an unsaturated hydrocarbon, then the OH radicalsbring about the addition reaction:

NMHC+OH→ROH  (15)

[0045] to form ROH radicals, which in turn react with oxygen as follows:

ROH+O₂→ROHO₂  (16)

[0046] whereby hydroxyl substituted RO₂ radicals are formed.

[0047] In either case, eventually the HO₂ radicals are produced as shownby the equation (9), whereafter ozone is generated following theequations (10) to (12) as with carbon monoxide or methane.

[0048] Since a NMHC and OH radicals react much faster than do methane orcarbon monoxide, in the atmosphere in which both NMHCs and NOx existabundantly it can be stated that the concentration of NMHCs isdeterminative of the concentration of photochemically generated orforming ozone. In the urban atmosphere there exist both NOx and NMHCsabundantly. Further, the ozone concentration varies according to thetypes of NMHCs.

[0049] The condition in which the primary factor that controls the ozoneconcentration is the hydrocarbon concentration is referred to asHydrocarbon Limited Condition.

[0050] As for a clean atmosphere in which there is little NOx, the ozoneconcentration is determined by the rate of reaction from NO to NO₂ asshown by the equation (10). Hence, this atmospheric condition underwhich the generation of ozone is strongly dependent on the concentrationof NOx (=NO+NO₂) is referred to as NOx Limited Condition.

[0051] As described above, NMHCs, which are highly reactive with OHradicals act to accelerate the photochemical formation of ozone by aseries of radical chain reactions whose rate-determinative steps are thereactions between the OH radicals and the NMHCs. In other words, theconcentration of NMHCs determines the life of OH radicals in theatmosphere. Thus, assuming a given amount of OH radicals, the higher theNMHC concentration, the quicker will the OH radical concentrationdecrease, and the lower the NMHC concentration, the slower will the OHradical concentration decrease. It follows, therefore, that measuringthe change of the OH radical concentration with time allows the NMHCconcentration to be estimated. The life of OH radicals is estimated tobe about 1 second in the clean atmosphere but in the urban atmosphere itis estimated to be tens to hundreds of millisecond.

[0052] Mention is next made of a forming photochemical ozone strengthestimating method utilizing a pump and probe technique and an apparatusfor carrying out the method, in accordance with the present invention.

[0053]FIG. 1 is a diagram illustrating the makeup of an apparatus forestimating the strength of forming photochemical ozone based on a pumpand probe technique in accordance with the present invention.

[0054] In FIG. 1, there is shown a forming photochemical ozone strengthestimating apparatus of pump and probe type 1 which includes a pumpinglaser oscillator unit 2 comprising a Nd:YAG laser oscillator 2 a and a4^(th) harmonic generator 2 b; and a probing laser oscillator unit 3comprising a diode pumped Nd:YAG laser oscillator 3 a, a 2^(nd) harmonicgenerator 3 b, a variable wavelength dye laser oscillator 3 c and a2^(nd) harmonic generator 3 d. Here, the pumping laser oscillator unit 2is adapted to generate a pulsed pumping laser light 2 c of a wavelengthof 266 nm whereas the probing laser oscillator unit 3 is adapted togenerate a pulsed probing laser light 3 e of a wavelength of 281.9 nm.

[0055] The apparatus also includes a fluorescence detecting unit 4having a radical forming unit 5 and a probing laser light lead-in unit 6connected to it through its side wall regions. The radical forming unit5 comprises a pair of straight tubes 5 a and 5 b of nontransparentmaterial extending coaxially with each other and connected to thefluorescence detecting unit 4 through those side wall regions,respectively. The straight tube 5 a has a transparent window 5 cfastened to its outer end to hermetically seal the same and is alsoprovided in its outer end region with an air inlet port, 5 d, and thestraight tube 5 b has a transparent window 5 e fastened to its outer endto hermetically seal the same and is also provided in its outer endregion with an air outlet port 5 f, which is in turn provided with aflow control unit 5 g and a vacuum pump 5 h.

[0056] The probing laser light lead-in unit 6 comprises a pair ofstraight tubes 6 a and 6 b of nontransparent material extendingcoaxially with each other and connected to the fluorescence detectingunit 4 through its side wall regions mentioned above, respectively. Eachof the straight tubes 6 a and 6 b has a plurality of baffle plates 6 cfor permitting the passing light rays to propagate only axially thereofand also has a transparent window 6 d, 6 e fastened to its outer end tohermetically seal the same.

[0057] The radical forming unit 5 and the probing laser light lead-inunit 6 are connected to the side walls of the fluorescence detectingunit 4 transversely to each other so that their axes intersect at apoint in its inside.

[0058] The fluorescence detecting unit 4 is designed to collectfluorescence generated in a region where the axis of the radical formingunit 5 and the axis of the probing laser light lead-in unit 6 intersectand to this intersect region is provided with a condensing mirror 7,convex lenses 8 and a photo detector 9 for measuring the intensity ofthe condensed light, and a receptacle 10 made of nontransparent materialfor retaining therein the condensing mirror 7, the convex lenses 8 andthe photo detector 9 in a sealed state.

[0059] The forming photochemical ozone strength estimating apparatusbased on the pump and probe technique operates controlled by a controlsystem 20 as shown in FIG. 2. The control system 20 includes aprocessing and integrated control unit 21 adapted to furnish a systemclock generator 22 with a control signal 23 indicating a start ofmeasurement. The system clock generator 22 upon receipt of the controlsignal 23 is adapted to generate system clocks and count the systemclocks to generate a control signal each time each of preselected countsis reached in an order mentioned below.

[0060] First, the pumping laser oscillator unit 2 is furnished with acontrol signal 24 to issue a pumping laser light pulse 2 c (FIG. 1).Then, the probing laser oscillator unit 3 is furnished with a controlsignal 25 to issue a probing laser light pulse 3 e. The fluorescencedetecting unit 4 is furnished with a control signal 26 to measure afluorescence intensity in the form of an analog signal for a given timeperiod. Then, a detection signal A/D conversion unit 27 is furnishedwith a control signal 28 to convert the analog signal representing thefluorescent intensity measured by the fluorescence detecting unit 4 intoa digital signal, which is delivered to the processing and integratedcontrol unit 21.

[0061] A cycle is set of these steps starting with the step offurnishing the probing laser oscillator unit 3 with a control signal 25and ending with the step of furnishing the detection signal A/Dconversion unit 27 with a control signal 28 and this cycle is repeated agiven number of times. After the cycle is so repeated, the pumping laseroscillator unit 2 is again furnished with a control signal 24 whereafterthe cycle is again repeated. Thus, for example, the pumping laseroscillator unit 2 is repetitively furnished with control signal at acycle of 1 Hz and the probing laser oscillator unit 3 is repetitivelyfurnished with a control signal 25 at a cycle of 1 kHz.

[0062] The processing and integrated control unit 21 is responsive to asignal from a laser fluctuation compensation system 29 to furnish eachof the pumping laser oscillator unit 2 and the probing laser oscillatorunit 3 with a sequential control signal to maintain the laser lightintensity therefrom at a given value while furnishing the flow controlunit 5 g with a control signal 30 to maintain the air flow in theradical forming unit 5 at a given value.

[0063] Operating the forming photochemical ozone strength estimatingapparatus 1 utilizing the pump and probe technique in accordance withthe present invention allows the atmosphere 11 introduced through theair inlet port 5 d to flow through the radical forming unit 5 passage ina laminar flow whose flow velocity is controlled by the vacuum pump 5 hand the flow control unit 5 g at a speed of flow optimum formeasurement. The pumping laser light pulse 2 c is introduced through thetransparent window 5 c of the radical forming unit 5 to pass along itsaxis, and the atmosphere 11 in the radical forming unit 5 upon absorbingthe energy of the pulsed pumping laser light 2 c gives rise to oxygenatoms in their excited state O (¹D), which then react with water vaporin the atmosphere, thus forming OH radicals. The OH radicals react withair pollutants in the atmosphere, i. e., reactive hydrocarbons such asNMHCs and methane, and their concentration then gradually decreases. Theprobing laser light pulse 3 e is introduced through the transparentwindow 6 d of the probing laser light lead-in unit 6 to pass along itsaxis. The OH radicals in the atmosphere 11 in an area in thefluorescence detection unit 4 where the radical forming unit 5 and theprobing laser lead-in unit 6 intersect absorb the pulsed probing laserlight 3 e so that their electrons are excited, and then whentransitioned to their ground state emit a fluorescent light 12.

[0064] The fluorescent light 12 is collected by the condensing mirrorand lenses 7 and 8, and its intensity is measured by the PMT 9. Afterthe pulsed pumping laser light 2 c is pumped to irradiate in theatmosphere therewith, the pulsed probing laser light 3 e is irradiatedthe atmosphere therewith a plurality of times and each time the probingirradiation occurs the fluorescent intensity is measured to determineits change with time. Since the fluorescent intensity is proportional tothe concentration of the OH radicals, the life of the OH radicals or thechange of the OH radical concentration with time can be found fromdetermining the change of the fluorescent intensity with time.

[0065] As mentioned above in connection with the equations (13) to (16),OH radicals are consumed by their reaction with NMHCs. Thus, the morelarger concentration of NMHCs in the atmosphere, the faster change ofthe concentration of OH radicals with time, and the smallerconcentrations of NMHCs in the atmosphere, the slower change of theconcentration of OH radicals with time.

[0066] Also, since HO₂ radicals produced by the reaction of NMHCs withOH radicals cause photochemical ozone to form as mentioned hereinbeforein connection with the equations (10) to (12), it is possible toestimate the concentration or strength of the forming photochemicalozone from the magnitude of the change of the concentration or strengthof the OH radicals with time. To wit, for the atmosphere in which thereexist an abundance of NMHCs and also an abundance of NOx and wherephotochemical ozone forms under the hydrocarbon limited conditions asmentioned above, it is possible to estimate the strength of the formingphotochemical ozone from the magnitude of the change of the strength ofOH radicals with time.

[0067] In the method of the present invention, the rate-determiningreactions in the photochemical ozone forming reactions in an atmosphereare actually excited in this apparatus and the quantity corresponding tothe rate of the reaction is measured. It is therefore possible toestimate easily and precisely the strength of the forming photochemicalozone actually to be formed in the atmosphere, regardless of the typesof air pollutants. More precisely mentioned, for example, given asimilarity coefficient between the energy of a pulsed pumpingirradiating laser light to the solar energy in an actual environment, itis possible by using the coefficient to estimate the strength ofactually forming photochemical ozone from a change of the strength of OHradicals with time as determined by the method and apparatus of thepresent invention.

Industrial Applicability

[0068] It will be appreciated from the foregoing description thataccording to the present invention it is possible to estimate easily andaccurately the forming strength of photochemical ozone without measuringthe strength or concentration of each individual type of reactivehydrocarbons in order to predict the strength of forming photochemicalozone from the rates of their reaction with OH radicals. There is thusprovided in accordance with the present invention a method that is vitaland essential to performing the measurement of air pollution in an urbanarea, and further of air pollution in the global atmosphere with easeand precision, as well as an apparatus for carrying out the method.

What is claimed is:
 1. A method of estimating strength of forming photochemical ozone utilizing a pump and probe technique, characterized in that it comprises the steps of: irradiating atmosphere containing pollutants with a pumping laser light to form OH radicals therefrom; irradiating such formed OH radicals with a probing laser light to cause a fluorescent light to be emitted therefrom; measuring a change of intensity of the fluorescent light with time; and determining a concentration of the air pollutants from the measured change of intensity of the fluorescent light with time.
 2. A method of estimating strength of forming photochemical ozone utilizing a pump and probe technique as set forth in claim 1, characterized in that said pumping laser light is a pulsed laser light having its wavelength adapted to photolyze ozone in the atmosphere, thereby forming excited oxygen atoms O (¹D).
 3. A method of estimating strength of forming photochemical ozone utilizing a pump and probe technique as set forth in claim 1, characterized in that said OH radicals are formed by the reaction of the excited oxygen atoms O (¹D) with water vapor.
 4. A method of estimating strength of forming photochemical ozone utilizing a pump and probe technique as set forth in claim 1, characterized in that said probing laser light is a pulsed laser light having its wavelength adapted to excite electrons in the OH radicals.
 5. A method of estimating strength of forming photochemical ozone utilizing a pump and probe technique as set forth in claim 1, characterized in that said change of intensity of the fluorescent light with time is measured by irradiating said pumping laser light and then irradiating said probing laser light repetitively each with a given time interval, and measuring the intensity of the fluorescent light after each time said probing laser light is irradiated.
 6. An apparatus for estimating strength of forming photochemical ozone utilizing a pump and a probe means, characterized in that it comprises: a pumping laser light oscillator means; a probing laser light oscillator means; an OH radical forming means; a probing laser light lead-in means; a fluorescence detection means for measuring an intensity of fluorescence; and a control means for providing said pumping laser light oscillator means, said probing laser light oscillator means and said fluorescence detection means with timing control signals whereby they are co-operated.
 7. An apparatus for estimating strength of forming photochemical ozone utilizing a pump and a probe means as set forth in claim 6, characterized in that said radical forming means includes: a first and a second straight tube extending coaxially and each connected to a side wall of said fluorescence detection means, said first straight tube having an outer end sealed hermetically with a transparent window and having an air inlet port in a region of the outer end of said first straight tube, said second straight tube having an outer end sealed hermetically with a transparent window and having an air out let port in a region of the outer end of said second straight tube, whereby the atmosphere can be led into the radical forming means, flowing, and the pumping laser light, e. g., pulsed, can be introduced into the radical forming means, passing in the first and second straight tubes axially thereof whereby the OH radicals are allowed to form therein.
 8. An apparatus for estimating strength of forming photochemical ozone utilizing a pump and a probe means as set forth in claim 6, characterized in that said radical forming means is provided at said air outlet port with a flow control means and a vacuum pump whereby the atmosphere introduced into the radical forming means is allowed to flow in the first and second straight tubes at a controlled rate of flow.
 9. An apparatus for estimating strength of forming photochemical ozone utilizing a pump and a probe means as set forth in claim 6, characterized in that said probing laser lead-in means includes: a first and a second straight tube extending coaxially and each connected to a side wall of said fluorescence detection means, each of said first and second straight tubes having an outer end sealed hermetically with a transparent window and having a plurality of baffle plates, whereby the probing laser light, e. g., pulsed, when introduced into the first and second straight tubes may cause electrons in the OH radicals to be excited.
 10. An apparatus for estimating strength of forming photochemical ozone utilizing a pump and a probe means as set forth in claim 6, characterized in that said fluorescence detection means includes: a condensing mirror and convex lens means for collecting a fluorescent light generated in an area in which the first and second straight tubes of said radical forming means and the first and second straight tubes of said probing laser light lead-in means intersect axially, a photo detector for measuring an intensity of the collected fluorescent light, and a receptacle composed of a nontransparent material for retaining said condensing mirror and convex lens means and said photo detector in a sealed state.
 11. An apparatus for estimating strength of forming photochemical ozone utilizing a pump and a probe means as set forth in claim 6, characterized in that said control means includes a system clock means for providing a first timing signal whereby said pumping laser light oscillator means is operated to issue a pulsed pumping laser light, a second timing signal timed on the basis of the first timing signal whereby said probing laser light oscillator means is operated to issue a pulsed laser probing light repetitively with a given time interval, and a third timing signal whereby measurement by said photo detector is synchronized with each pulsed probing laser light issued.
 12. An apparatus for estimating strength of forming photochemical ozone utilizing a pump and a probe means as set forth in claim 6, characterized in that said pumping laser light oscillator means is adapted to issue a pulsed laser light having a wavelength of 266 nm repetitively at a repetition rate of 1 Hz.
 13. An apparatus for estimating strength of forming photochemical ozone utilizing a pump and a probe means as set forth in claim 6, characterized in that said probing laser light oscillator means is adapted to issue a pulsed laser light having a wavelength of 281.9 nm repetitively at a repetition rate of 1 kHz.
 14. An apparatus for estimating strength of forming photochemical ozone utilizing a pump and a probe means as set forth in claim 6, characterized in that said photo detector comprises a PMT (photomultiplier tube). 